Compressed instruction format

ABSTRACT

A technique for decoding an instruction in a variable-length instruction set. In one embodiment, an instruction encoding is described, in which legacy, present, and future instruction set extensions are supported, and increased functionality is provided, without expanding the code size and, in some cases, reducing the code size.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relates to microprocessors andmicroprocessor systems. In particular, at least one embodiment of theinvention relates to an instruction encoding technique resulting in acompressed instruction format.

2. Description of Related Art

Microprocessor technology has evolved over the years at a fast rate.Advances in computer architecture and semiconductor technology havecreated many opportunities to design new processors. There are typicallytwo options for designing new processors: (1) defining a completely newarchitecture, and (2) extending the current architecture to accommodatenew features.

New processors involve new features in both hardware and software. A newprocessor based on existing design typically has an additional set ofinstructions that can take advantage of the new hardware design.However, extending an instruction set by adding a new set ofinstructions is a challenging problem because of the constraints in theencoding of the instructions and in storing instruction opcode maps.Therefore there is a need in the industry to provide an efficient methodfor extending an instruction set without increasing hardware complexityand without significantly expanding the number and size of opcode maps.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will become apparent fromthe following detailed description of the invention in which:

FIG. 1 is a block diagram illustrating a processing system, in which atleast one embodiment may be used.

FIG. 2 is a block diagram illustrating an instruction format which mayinclude fields represented by one embodiment.

FIG. 3 is a block diagram illustrating logic to decode an instruction,in accordance with at least one embodiment.

FIG. 4 is a diagram illustrating a first phase decoder, according to oneembodiment.

FIG. 5 is a diagram illustrating a second phase decoder, according toone embodiment.

FIG. 6 is a flow diagram illustrating operations to perform oneembodiment.

DESCRIPTION OF THE PRESENT INVENTION

At least one method, apparatus and system for extending an instructionset using embodiments of a unique instruction attribute encoding.Disclosed embodiments further enable an instruction set architecture toexpand while conserving storage area for storing opcode maps associatedwith the instruction set architecture. In at least one embodiment, a setof existing instruction fields are mapped, compressed, or replaced byinstruction attribute fields that may retain information associated withprior art instruction fields, but may result in a more compactinstruction requiring less storage area for storing the instructions andinstruction opcode maps. In one embodiment, instruction attributeencodings may enable further instruction and micro-architecturalfeatures, including an extra source operand to be used by someinstructions as well as an increased data size for instructions, such asvector instructions (e.g., single-instruction-multiple-data (SIMD)instructions).

In the following description, for purposes of explanation, numerousspecific details such as processor types, instruction formats, logicgate types, and escape opcode values are set forth in order to provide athorough understanding of embodiments of the invention. However, it willbe apparent to one skilled in the art that these specific details arenot required in order to practice embodiments the invention. In otherinstances, well-known electrical structures and circuits are shown inblock diagram form in order not to obscure embodiments of the invention.In the following description, the notation ‘0x’ indicates the numberthat follows is in hexadecimal format.

Reference to FIG. 1 illustrates a processing system 100, in which atleast one embodiment of the invention may be used. System 100 may beused, for example, to decode and execute new instruction types 118 b orexisting instructions 118 a in instruction set 118. In one embodiment, aprocessing system includes any system that has a processor 110, such as,for example; a digital signal processor (DSP), a microcontroller, anapplication specific integrated circuit (ASIC), or a microprocessor. Theprocessor 110 may be of any type of architecture, such as complexinstruction set computers (CISC), reduced instruction set computers(RISC), very long instruction word (VLIW), multi-threaded or hybridarchitecture. Processing systems may include personal computers (PCs),engineering workstations, personal digital assistants and otherhand-held devices, set-top boxes and the like).

FIG. 1 illustrates that the processor 110 includes a decode unit 116, aset of registers 114, at least one execution unit 112, and at least oneinternal bus 111 for executing instructions. Of course, the processor110 contains additional circuitry, which is not necessary tounderstanding the invention. The decode unit 116, registers 114 andexecution unit 112 are coupled together by one or more internal bus 111.The decode unit 116 is used for decoding instructions received byprocessor 110 into control signals and/or microcode entry points. Theinstructions may be issued to the decode unit 116 by an instructionbuffer (such as, e.g., 310 in FIG. 3). In response to these controlsignals and/or microcode entry points, the execution unit 112 performsthe appropriate operations. The decode unit 116 may be implemented usingany number of different mechanisms (e.g., a look-up table, a hardwareimplementation, a programmable logic array (“PLA”), etc.).

The decode unit 116 is shown to be capable of decoding instructions 106that follow formats defined by an extended instruction set 118. Theinstruction set 118 includes an existing instruction set 118 a and a newinstruction set 118 b, which may represent any new instructions that maybe introduced to the architecture. The instruction set 118 includesinstructions for performing operations on scalar and packed (or vector)data. The format for these operations can be any convenient format,including single-precision, double-precision, and extendedfloating-point numbers, signed and unsigned integers, and non-numericdata. For at least one embodiment, the instructions defined in theinstruction set 118 may vary in length from one another.

Instructions 106, which follow the formats set forth by the instructionset 118, may be stored in a memory system 102. Memory system 102 isintended as a generalized representation of memory or memory hierarchiesand may include a variety of forms of memory, such as a hard drive,CD-ROM, random access memory (RAM), dynamic random access memory (DRAM),static random access memory (SRAM), flash memory and related circuitry.Memory system 102 may store, in addition to instructions 106, data 104represented by data signals that may be executed by processor 110.

Instruction Format

FIG. 2 is a diagram illustrating a format of an instruction 200 whichmay include fields represented by one embodiment of the invention. Theinstruction format 200 includes a prefix field 210, an opcode field 220,and operand specifier fields (e.g., modR/M, scale-index-base,displacement, immediate, etc.). The operand specifier fields may beoptional and include a modR/M field 230, an SIB field 240, adisplacement field 250, and an immediate field 260.

In one embodiment, the term “escape opcode” refers to a two-byte valuethat indicates to decoder logic that an opcode to which an instructioncorresponds may have various attributes, such as a three-byte opcode, inwhich two bytes correspond to the escape opcode and a third bytecorresponds to an opcode indicating the function of the instruction. Forexample, in one embodiment, the two-byte value in the three-byte escapefield may be one of the following values: 0x0F38, 0x0F39, 0x0F3A or0x0F3B. The contents of the modR/M field 230 indicate anaddressing-form. The modR/M field may specify registers and addressingmodes.

Certain encodings of information in the modR/M field 230 may indicatethat a second byte of addressing information is present in the SIB(Scale/Index/Base) field 240 to fully specify the addressing form of theinstruction code. For instance, a base-plus-index addressing form and ascale-plus-index addressing form may each include information, such asscale factor, register number for the index, and/or register number ofthe base register, in the SIB field 240.

One skilled in the art will recognize that the format 200 set forth inFIG. 2 is illustrative, and that other organizations of data within aninstruction code may be utilized with disclosed embodiments. Forexample, the fields 210, 220, 230, 240, 250, 260 need not be organizedin the order shown, but may be re-organized into other locations withrespect to each other and need not be contiguous. Also, the fieldlengths discussed herein should not be taken to be limiting. A fielddiscussed as being a particular member of bytes may, in alternativeembodiments, be implemented as a larger or smaller field. Also, the term“byte,” while used herein to refer to an eight-bit grouping, may inother embodiments be implemented as a grouping of any other size,including 4 bits, 16 bits, and 32 bits.

As used herein, an instruction (such as one of the instructions 106illustrated in FIG. 1) includes certain values in the fields of theinstruction format 200 shown in FIG. 2. Such an instruction is sometimesreferred to as “an actual instruction.” The bit values for an actualinstruction are sometimes referred to collectively herein as an“instruction code.”

The acceptable byte values for an actual instruction are specified inthe instruction set 118 (FIG. 1). Acceptable byte values for each of thefields of the instruction format 200 are those values that decode logic,such as instruction length decoder 306 (FIG. 3) and decode unit 116(FIG. 1), recognize and operate upon to generate decoded instructioncode. For each instruction code, the corresponding decoded instructioncode uniquely represents an operation to be performed by the executionunit 112 (FIG. 1) responsive to the instruction code. The decodedinstruction code may include one or more micro-operations.

The prefix field 210 illustrated in FIG. 2 may include a number ofprefixes. In one embodiment, the prefix field 210 includes up to fourprefixes, with each prefix being one byte. In some applications, such asfor an extended instruction set, the prefix field may be used to extendthe opcode instruction space. Furthermore, in at least one embodiment,the prefix may be encoded to provide as much or more information aboutthe instruction as in the prior art, but requiring less storage areathan instructions using some prior art prefix encodings.

The contents of the opcode field 220 specify one or more operations tobe performed by a processor executing the instruction. For at least oneembodiment, the opcode field may include three bytes. In otherembodiments, the opcode may include one, two or more than three bytes.For some instructions, the three-byte escape opcode value in thetwo-byte field 118 c of the opcode field 220 is combined with thecontents of a third byte 225 of the opcode field 220 to specify anoperation. This third byte 225 is referenced to herein as aninstruction-specific opcode. For some instructions, the three-byteescape opcode value in the two-byte field 118 c of the opcode field 220is combined with the contents of the prefix field 210 and the contentsof the instruction-specific opcode field 225 of the opcode field 220 tospecify an operation.

In general, the combination of the prefix field 210 and the opcode field220 creates a number of different types of instructions. Forillustrative purposes, FIG. 2 shows only seven types of instructions: aregular one-byte instruction 212, a regular instruction with prefix asqualifier 214, a regular escape instruction 216, a first extendedinstruction type 222, a second extended instruction type 224, a firstnew instruction type 226 and a second new instruction type 228. As isknown by one skilled in the art, other types of instruction can besimilarly defined. In one extended instruction type 226, a REX field isincluded to indicate the registers to be used with the instruction. Forexample, one instruction type indicated by the REX enables additionalregisters in 64-bit mode.

The regular one-byte instruction 212 includes regular instructions withone-byte instruction-specific opcodes in the opcode field 220. Theregular instruction with prefix as qualifier 214 includes regularinstructions that use the prefix as a qualifier for the opcode. Forexample, a string instruction may use a REPEAT prefix value to repeatthe string instruction by a number of times specified in the countregister or until a certain condition is met. The prefix value used ininstruction 214 does not add a completely new meaning to the opcodevalue that follows in the opcode field 220. Rather, the prefix value ismerely used as a qualifier to qualify the opcode with additionalconditions. The use of the prefix in the instruction 214 may bedifferent for different instruction types.

In one embodiment, the regular escape instruction 216 is a regularinstruction that utilizes an escape opcode in a first field 215 of theopcode field 220 to indicate to decoder hardware that aninstruction-specific opcode in a second field 217 of the opcode field220 is used to define the instruction. For example, in one embodiment, afloating-point coprocessor escape opcode value 0xD8 through 0xDF in thefirst byte 215 of the opcode field 220 indicates that the opcode valuethat follows in the second byte 217 of the opcode field 220 should beinterpreted as a coprocessor instruction and should be directed tocoprocessor hardware for execution.

The first extended instruction type 222 is a particular type of escapeinstruction that is defined to contain a predefined escape opcode value,0x0F, in a first field 221 of the opcode field 220. The escape opcode0x0F indicates to decoder hardware that an instruction-specific opcodevalue in a second field 223 of the opcode field 220 is used to definethe instruction. Instructions of the first extended instruction type 222may, depending on the value of the second opcode byte (and, in somecases, the value of three bits of the modR/M field 230), be of varyinglengths. For example, two instructions (Packed Shift Left Logical) ofextended instruction type 222 may include the following instructionfield values, respectively:

PSLLW (shift value in register): 0F:F1:1b“11xxxyyy”, where xxx defines afirst register and yyy defines a second register

PSLLW (shift by immed. value): 0F:71:1b“11110xxx”: immed data (8 bits),where xxx defines a register

In each of the PSSLW instructions set forth, the first word of theopcode field 220 includes the escape value 0x0F. The first instructionis three bytes long but the second instruction is four bytes because itincludes a byte of immediate data. Accordingly, for extended instructiontype 222 decoder hardware (such as, for example, instruction lengthdecoder 306 in FIG. 3) utilizes the escape opcode value 0x0F in thefirst field 221 of the opcode field 220 as well as the value in thesecond byte 223 of the two-byte opcode field 220 and the value of modR/Mfield 230 to decode the length of the instruction.

Examples of Instruction Prefixes and Escape Opcodes

Instruction prefixes were originally developed to enhance a set ofinstructions. For example, the repeat prefix is developed to repeat astring instruction. The repeat prefix codes are 0xF3 (REP, REPE) and0xF2 (REPNE). The prefix used as such does not define a new meaning forthe opcode that follows. It merely defines additional operationalconditions for the opcode.

Escape opcodes provide an extension to the instruction set. For example,the escape opcodes 0xD8 through 0xDF are used to indicate that thesecond opcode byte 217 contains a value defining an instruction for thefloating-point unit. The decoder passes the opcode to the floating-pointdecoder.

For at least one instruction set, a 3-byte escape opcode is a two-byteentity having a value of 0x0F3y, where y is 0x8, 0x9, 0xA or 0xB. The3-byte escape opcode value in the 3-byte escape code field 118 cindicates to the decoder that the instruction-specific opcode value inthe third byte 225 of the opcode field 200 indicates an instruction in anew instruction set. A new instruction set may be any new instructionset added to the architecture.

In contrast to a 2-byte escape opcode, the value in the 3-byte escapeopcode field 118 c indicates to the decoder the method to be used todetermine the length of the instruction. That is, each value for the3-byte escape opcode is associated with a particular method to be usedto determine the instruction length for every instruction in the mapcorresponding to the particular 3-byte escape code. For instance, thevalue 0x0F38 in the 3-byte escape opcode field 118 c is associated withan associated opcode map. The length for each instruction in the 0x0F38opcode map may be calculated using the same length-determination methodused to determine the length of the other instructions in the 0x0F38opcode map.

Similarly, the length of each instruction of the respective opcode mapsassociated with the remaining 3-byte escape opcode values (0x0F39,0x0F3A, 0x0F3B) may be calculated with the same length-determinationlogic used to determine the length of the other instructions in therespective opcode map.

The length-determination logic used to determine the length ofinstructions for each instruction in one of the new opcode maps issimplified in that the same set of input terms is evaluated to determinethe length of each instruction in the opcode map. Suchlength-determination logic is referred to herein as a “fixed-input”logic or method. That is, each input term evaluated to determine thelength of one instruction in the map is also relevant to determinationof the length of every other instruction in the map. The fixed set ofterms to be evaluated may differ from opcode map to opcode map. Whilethe set of inputs to be evaluated may differ among opcode maps, theinputs evaluated to determine instruction length are the same across allinstructions in a give 3-byte opcode map.

The combination of a prefix and an escape opcode provides a significantenlargement of a processor's opcode table to allow additional newinstruction sets. This combination uses the existing prefix codes todefine a new set of instructions, in addition to the instruction setcreated by the escape opcodes. By using the existing prefix codes, thedecoding circuitry for the existing instruction set may remainrelatively unmodified to support decoding of the new instructions 118 c(FIG. 1).

The instruction-specific opcode values (in the third byte 225 of theopcode field 220) of some or all of the new instructions may be the sameas the opcodes of the existing instructions. By using the same opcodeswith the prefix and escape opcodes to define a new set of instructions,the decoding circuitry may be less complex than having a completely newset of opcodes for the new instruction set.

In one embodiment, the prefix value is used to define new instructions.Furthermore, prefixes can still be used in the traditional role ofenhancing the opcode or qualifying the opcode under some operationalcondition.

Table 1, below, illustrates some prior art examples of an instructionset using prefixes and three-byte escape opcodes.

TABLE 1 (Prefix)/Escape Opcode/ Instruction-specific Instruction opcode(in hex) Definition PHADDW 0F 38 01/r Add horizontally packed numbersfrom 64-bit register or memory to 64-bit register PHADDW (66)0F 38 01/rAdd horizontally packed numbers from 128-bit register or memory to 128-bit register PHADDD 0F 38 02/r Add horizontally packed numbers from64-bit register or memory to 64-bit register PHADDD (66)0F 38 02/r Addhorizontally packed numbers from 128-bit register or memory to 128- bitregister PHADDSW 0F 38 03/r Add horizontally packed numbers withsaturation from 64-bit register or memory to 64-bit register PHADDSW(66)0F 38 03/r Add horizontally packed numbers with saturation from128-bit register or memory to 128-bit register PHSUBW 0F 38 05/rSubtract horizontally packed signed words in 64- bit register or memoryto 64-bit register PHSUBW (66)0F 38 05/r Subtract horizontally packedsigned words in 128-bit register or memory to 128-bit register PHSUBD 0F38 06/r Subtract horizontally packed signed double words in 64-bitregister or memory to 64-bit register PHSUBD (66)0F 38 06/r Subtracthorizontally packed signed double words in 128-bit register or memory to128-bit register PHSUBSW 0F 38 07/r Subtract horizontally packed signedwords in 64- bit register or memory to 64-bit register as saturatedresult PHSUBSW (66)0F 38 07/r Subtract horizontally packed signed wordsin 128-bit register or memory to 128-bit register as saturated resultPMADDUBSW 0F 38 04/r Multiply and add packed signed and unsigned numberin 64-bit register or memory to 64-bit register PMADDUBSW (66)0F 38 04/rMultiply and add packed signed and unsigned number in 128-bit registeror memory to 128-bit register PMULHRSW 0F 38 0B/r Packed multiply highwith round and scaling from 64- bit register or memory to 64-bitregister PMULHRSW (66)0F 38 0B/r Packed multiply high with round andscaling from 128- bit register or memory to 128-bit register PSHUFB 0F38 050/r Packed shuffle bytes in 64- bit register or memory to 64-bitregister PSHUFB (66)0F 38 00/r Packed shuffle bytes in 128- bit registeror memory to 128-bit register PSIGNB 0F 38 08/r Packed sign byte 64-bitregister or memory to 64-bit register PSIGNB (66)0F 38 08/r Packed signbyte 128-bit register or memory to 128- bit register PSIGNW 0F 38 09/rPacked sign word 64-bit register or memory to 64-bit register PSIGNW(66)0F 38 09/r Packed sign word 128-bit register or memory to 128- bitregister PSIGND 0F 38 0A/r Packed sign double word 64-bit register ormemory to 64-bit register PSIGND (66)0F 38 0A/r Packed sign double word128-bit register or memory to 128-bit register PSRMRG 0F 3A 0F/r Packshifted right and merge contents of 64-bit register or memory to 64-bitregister PSRMRG (66)0F 3A 0F/r Pack shifted right and merge contents of128-bit register or memory to 128- bit register PABSB 0F 38 1C/r Packedbyte absolute value of value in 64-bit register or memory to 64-bitregister as unsigned result PABSB (66)0F 38 1C/r Packed byte absolutevalue of value in 128-bit register or memory to 128-bit register asunsigned result PABSW 0F 38 1D/r Packed word absolute value of value in64-bit register or memory to 64-bit register as unsigned result PABSW(66)0F 38 1D/r Packed word absolute value of value in 128-bit registeror memory to 128-bit register as unsigned result PASBSD 0F 38 1E/rPacked double word absolute value of value in 64-bit register or memoryto 64-bit register as unsigned result PABSD (66)0F 38 1E/r Packed doubleword absolute value of value in 128-bit register or memory to 128-bitregister as unsigned result

In the above examples, the instructions with the prefix 0x66 relate toinstructions that utilize one or more extended-size registers (such as128-bit register size), while the instructions without the prefix 0x66relate to instructions that utilize one or more smaller-size registers(such as 64-bit register size). The smaller-size registers are referredto herein as “regular length” registers. As is known by one skilled inthe art, the exact codes for prefixes are implementation-dependent andthe 0x66 prefix value discussed above is merely for illustrativepurposes.

The original x86 instruction set provided for a 1-byte opcode withvarious formats of address syllable and immediate operand contained inadditional bytes whose presence was known from the first “opcode” byte.Additionally, there were certain byte values that were reserved asmodifiers to the opcode (called prefixes, as they had to be placedbefore the instruction). When the original palette of 256 opcode bytes(including these special prefix values) was exhausted, a single byte wasdedicated as an escape to a new set of 256 opcodes. As vectorinstructions (e.g., SIMD) were added, a need for more opcodes wasgenerated, and the “two byte” opcode map also was insufficient, evenwhen expanded through the use of prefixes. To this end, new instructionswere added in additional maps which use 2 bytes plus an optional prefixas an identifier.

Additionally, in order to facilitate additional registers in 64-bitmode, an additional prefix may be used (called “REX”) in between theprefixes and the opcode (and any escape bytes necessary to determine theopcode). In one embodiment, the REX may have 4 “payload” bits toindicate use of additional registers in 64-bit mode. In otherembodiments it may have fewer or more than 4 bits. The general format ofat least one instruction set is illustrated generically by thefollowing:

-   -   [prefix] [rex] escape [escape2] opcode (etc.)

In this example “etc.” represents the address syllable (e.g., one to sixbytes) and perhaps additional immediate information for the instruction.

As new instruction sets are added, it can be seen that the size ofinstructions, including prefix, data-size indicator, escape codes,opcodes, and intermediate, address fields, etc., are causing the codesize to expand, which can be undesirable for a number of reasons.

In order to enable new instruction set extensions, while remainingbackward compatible with existing and legacy instruction sets, a newinstruction encoding technique may be desirable. Accordingly, at leastone embodiment of the invention includes an instruction encoding thatenables backward compatibility, enhanced functionality, while reducingcode size. In one embodiment, the instruction encoding uses a new byteto indicate whether the instruction requires a second extension field ornot. The following illustration shows a legacy instruction and how atleast one embodiment of the invention at least maintains if not improvesthe code size to which the instruction contributes. In the exampleillustrated below, the legacy escape field is replaced with a new escapefield, legacy prefixes are partially compressed as part of a “payload”byte, legacy prefixes may be reclaimed and made available for futureinstruction expansion, and new features are added (e.g., increasedvector length increase and additional source register specifier.

Opcode which DOES NOT include an “escape2”

As can be seen above, the non-fixed part of the instruction (prefix,REX, escape code) has gone from a range of 1 to 3 bytes to 2 bytes. Inthe worst case, the instruction encoding according to one embodimentadds only one byte the legacy encoding, and in the best case, it savesone byte of encoding over the legacy encoding. In the above example, thebyte-long prefix, “[66, F2, F3]”, is represented by two bits “pp”, afield of the REX byte is represented by one bit, “R”, and the escapebyte is re-coded from “0F” to “C5” (which will be discussed below).Furthermore, a 4-bit field, “VVVV” has been added to specify an extrasource operand, and an increased vector size has been provided forthrough vector size bit “L”, which may designate either a 128 bit or a256 bit vector size.

The following illustrates an embodiment using two bytes to encode aninstruction set instruction, which may be used when a second escape codeis present in the original instruction, or when extra bits (e.g, the XBand W fields) in the REX field need to be used. In the embodimentillustrated below, legacy Escape is represented by a new escape value,legacy prefixes are fully compressed as part of the “payload” bytes,legacy prefixes are reclaimed and available for future expansion, thesecond escape code is compressed in a “map” field, with future map orfeature space available, and new features are added (e.g., increasedvector length and an additional source register specifier).

Opcode which optionally includes an “escape2”

In the above illustration, the original instruction is assumed to useeither the REX or the “escape2” fields (or both), and as a result thelength of the non-fixed part of the instruction has gone from a range of2 to 4 bytes to 3 bytes. In the worst case, one embodiment of theinvention increases the original instruction encoding by 1 byte, in thebest case, the embodiment decreases the original instruction encoding by1 byte. In contrast to the single escape code example mentioned before,the REX byte is represented by 4 bits (RXB and W) and the second escapefield is present and encoded in 5 bits (“mmmmm”) instead of using adiscrete byte as in the prior art encoding.

Although embodiments disclosed use 5 bits to compress up to 32 differentvalues of escape2 code bytes, in other embodiments, bits from theescape2 code bits may be used for other functions, such as extending thevector length to 4 combinations (illustrated below, where the “mmmmm”bits are renamed as ‘L’) and maintaining the ability to encode 16different escapes.

C4 RXB.mmmmm W.VVVV.Lpp C4 RXB.L'.mmmm W.VVVV.Lpp

Similarly, if only 4 values of escape2 code are needed, then the upper“mmmmm” bits could be renamed to indicate the QRS feature, which mayassume seven values other than the original “default behavior” valuecompatible with the original implementation.

C4 RXB.QRS.mm W.VVVV.Lpp

In the above examples, the escape code is “C4”, indicating that theinstruction is represented by the second encoding rather than onementioned before, indicated by “C5”. It should be noted that the choiceof values “C5” and “C4” values and the encoding of the payloads ischosen so that it will be backwards compatible with unimplementedopcodes on current processors of at least one architecture (e.g., x86)One of ordinary skill in the art will recognize that C5 and C4correspond to the “LDS” and “LES” instructions, respectively, which arelegal under opcodes in an x86 architecture.

In some embodiments, instructions in the format, “[prefixes] [REX][escape bytes] opcode [additional bytes to end of instruction]”, may betranslated into a more compact and efficient format, “[Attributes][opcode] [additional bytes to end of instruction]”, where the“Attributes” field may include fewer bits than the original instructionfrom which it was translated.

In addition to instruction sets mentioned herein, other instructionsfrom other instruction sets may also benefit from embodiments of theinvention.

Instruction Decoding

FIG. 3 is a diagram illustrating a logic 300 to decode variable-lengthinstructions. The logic 300 may include an instruction length decoder306, an instruction rotator 308, an instruction buffer 310, a firstpre-decode logic 320, a second pre-decode logic 330, and an opcodedecoder 340. The first phase pre-decode logic 320, the second phasepre-decode logic 330, and the opcode decoder 340 form all or part of thedecode unit 116 illustrated in FIG. 1. While illustrated as a singleentity, any or each of first phase pre-decode logic 320, the secondphase pre-decode logic 330, and the opcode decoder 340 may beimplemented as separate logic blocks.

The instruction length decoder 306 determines the length of an actualinstruction code that has been fetched from external memory (such as,e.g., memory 102, FIG. 2). For illustrative purposes, an instructioncode is assumed to include up to five bytes: the first byte correspondsto I_(N) to I_(N+7), the second byte corresponds to I_(K) to I_(K+7),the third byte corresponds to I_(L) to I_(L+7), the fourth bytecorresponds to I_(M) to I_(M+7), and the fifth byte corresponds to I_(P)to I_(P+7), where I_(N) to I_(N+7), I_(K) to I_(K+7), I_(M) to I_(M+7),I_(L) to I_(L+7), and I_(P) to I_(P+7) refer to the bit positions of theinstruction code. In practice, however, an actual instruction mayinclude more than five bytes in its instruction code. Similarly, anactual instruction may include less than five bytes in its instructioncode.

For at least one embodiment, the five illustrates bytes of theinstruction code are contiguous, such that K=N+8, L=K+8 and L=N+16, andM=L+8, M=K+16 and M=K+24, and so on. However, as is discussed above inconnection with FIG. 2, the fields of the format 200 illustrated in FIG.2 need not occupy the positions shown. Accordingly, the illustrativefive bytes of an instruction code that are discussed herein may be inany order and need not be contiguous.

One of skill in the art will recognize that logic of the instructionlength decoder 306 may implement fairly complex length decode methods ina system that supports variable-length instructions. This is especiallytrue in systems that require different methods, that evaluate differentinputs, to determine instruction length for instructions within the sameopcode map. In some embodiments, the length of each instruction withinan opcode map is determined by a single fixed-input length-determinationlogic.

The rotator 308 rotates the raw instruction bytes such that the firstbyte to be decoded is in an initial position. The rotator 308 thusidentifies the beginning of the instruction bytes to be decoded. Itshould be noted that, although the rotator 308 may identify the firstbyte of an instruction, such as a prefix byte, the first byte need notbe identified.

The instruction buffer 310 receives and stores the instructions thathave been fetched from the external memory. For at least one embodiment,the instructions are length-decoded and rotated before being received bythe instruction buffer 310. For at least one embodiment, the instructionbuffer 310 is implemented as an instruction cache.

The first phase instruction pre-decoder 320 receives the instructionbytes and detects the presence of one or more of a set of predefinedprefixes and/or escape codes and/or opcodes used as part of aninstruction set. The second phase pre-decode logic 330 utilizes theresults of the pre-decoder 320 to translate the instruction informationreceived from a prior art instruction format to one in accordance withone embodiment. The translated instruction is then sent to the opcodedecoder 340 to generate instruction signals (e.g., micro-operations)consistent with the desired instruction operation.

In one embodiment, the translation logic 340 translates an instructionof format, “[prefixes] [REX] [escape bytes] opcode [additional bytes toend of instruction]”, into a more compact and efficient format,“[Attributes] [opcode] [additional bytes to end of instruction]”. Inother embodiments, other instructions having other instruction formatsare translated into the above compact format.

FIG. 4 is a block diagram illustrating at least one embodiment of afirst phase pre-decoder logic 420. In one embodiment, the pre-decoder420 receives a fetched cacheline and produces information for byteswithin the instruction. For instance, if a byte of the receivedinstruction contains 0x66 the pre-decoder logic 420 may realize thefollowing algorithm: If 0x66 is located within the prefix portion of theinstruction, otherwise, if 0x66 is located in the opcode byte andpreceded by 0x0F then the 0x0F is an escape code and the opcodecorresponds to a first opcode map entry, otherwise if 0x66 is located inthe opcode byte and not preceded by 0x0F then there is no escape code inthe instruction and opcode corresponds to a second opcode map entry, if0x66 is in the modr/m (address specifier) field in 32-bit mode, then0x66 has a four byte displacement, etc.

As illustrated in FIG. 4, the pre-decoder logic 420 may include parallellogic, implemented in programmable logic arrays 421, 423 425, etc.,enabled to decode corresponding portions of an instruction, such as theprefix, escape codes, opcodes, etc. Each portion 421-425, in oneembodiment may interpret its respective instruction portion as meaningvarious things, according to the value in the corresponding instructionportion, as illustrated for logic array 421 in FIG. 4. After eachportion of the instruction has been pre-decoded by logic 420 todetermine what each portion means, the instruction may then be passed toa second pre-decode phase (illustrated as 320 in FIG. 3) to translatethe instruction into a more efficient and compact encoding scheme,according to one embodiment.

FIG. 5 illustrates a second phase pre-decode logic 430 to translate aninstruction into a format consistent with at least one embodiment of theinvention. For each opcode pre-decoded by the first pre-decode logicillustrated in FIG. 4, an attribute field 431, opcode field 433, andaddress/data/intermediate/etc field 435 is decoded and sent to opcodedecoder logic (illustrated as 340 in FIG. 3) to decode the opcode intosignals to carry out the operations prescribed by the opcode. In oneembodiment, attribute field 431 may include bits to indicate theinformation described previously, including prior art opcode mapinformation (e.g., escape codes), REX information, and prefixinformation, as well as new information according to one embodiment,such as an extra source register address and an expanded vector size.

FIG. 6 is a flowchart illustrating operations to perform an instructiondecoding and translation technique according to at least one embodiment.At operation 601, instruction bytes are rotated such that the first byteto be decoded is in an initial position. At operation 605, aninstruction buffer receives and stores the instructions that have beenfetched from the external memory. At operation 610, the instruction ispre-decoded in a first phase, in which the presence of one or more of aset of predefined prefixes and/or escape codes and/or opcodes used aspart of an instruction set is detected. At operation 615, a second phasepre-decoding of the instruction translates the instruction informationreceived from a prior art instruction format to a format in accordancewith one embodiment of the invention. At operation 620, the translatedinstruction is then sent to the opcode decoder to generate instructionsignals consistent with the desired instruction operation.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from the presentinvention in its broader aspects. The appended claims are to encompasswithin their scope all such changes and modifications that fall withinthe true scope of the present invention.

1. An apparatus comprising: a first instruction decode logic circuitryto indicate to a second instruction decode logic circuitry the functionof each of a plurality of fields within an instruction, wherein thesecond instruction decode logic circuitry is to use the function of eachof the plurality of fields indicated by the first instruction decodelogic circuitry to translate the instruction from a first instructionset format to a second instruction set format of an extended instructionset, wherein the second instruction set format is to represent at leastone of the plurality of fields with fewer bits than the firstinstruction set format of the instruction.
 2. The apparatus of claim 1further comprising an opcode decode logic circuitry to decode an opcodewithin the instruction into one or more signals to indicate one or moreoperations to be performed by executing the instruction.
 3. Theapparatus of claim 1, wherein the second instruction set format of theinstruction includes an attribute field and an opcode field.
 4. Theapparatus of claim 3, wherein the attribute field is to indicate anopcode map, a REX value, and a prefix value.
 5. The apparatus of claim4, wherein the attribute field of the second instruction set format ofthe instruction is to include fewer bits than the sum of the bits of thecorresponding fields of the first format.
 6. An apparatus comprising: afirst instruction decode logic circuitry to indicate to a secondinstruction decode logic circuitry the function of each of a pluralityof fields within an instruction, wherein the second instruction decodelogic circuitry is to use the function of each of the plurality offields indicated by the first instruction decode logic circuitry totranslate the instruction from a first format to a second format,wherein, the second format is to represent at least one of the pluralityof fields with fewer bits than the first format of the instruction, thesecond format of the instruction includes an attribute field and anopcode field, the attribute field is to indicate an opcode map, a REXvalue, and a prefix value and in the attribute field, the opcode map iseither represented by a first or second value, the REX value isindicated by either one bit or four bits, depending on the value of theopcode map, and the prefix value is to be represented by two bits. 7.The apparatus of claim 6, wherein the second format of the instructionincludes a source field to indicate a third source register thisdifferent from two other source registers indicated by the instruction.8. The apparatus of claim 7, wherein the second format of theinstruction includes a field to indicate a first and second vector size.9. A non-transitory machine readable medium having stored thereon aninstruction having a first instruction format, which if decoded by amachine causes the machine to generate a result comprising: arepresentation of the instruction having an extended instruction setformat that is different than the first instruction format, wherein therepresentation of the instruction retains all information included inthe first instruction format using fewer bits, and includes an attributefield having an opcode map field, a REX field, a third source registerfield, a vector size field, and a prefix field.
 10. The machine-readablemedium of claim 9, wherein the opcode map field is one byte in length,the REX field is one bit in length, the third source register field isfour bits in length, the vector size field is one bit in length, and theprefix field is two bits in length.
 11. The machine-readable medium ofclaim 9, wherein the opcode map field is one byte in length, the REXfield is four bits in length, the third source register field is fourbits in length, the vector size field is one bit in length, and theprefix field is two bits in length.
 12. The machine readable medium ofclaim 11, further including a prefix field of five bits in length. 13.The machine-readable medium of claim 9, wherein the representation ofthe instruction is smaller in bit length than the instruction.
 14. Themachine-readable medium of claim 9, wherein the representation of theinstruction is the same size in bit length as the instruction.
 15. Themachine-readable medium of claim 9, wherein the opcode map fieldincludes either 0xC4 or 0xC5, depending on the instruction type.
 16. Asystem comprising: a memory to store an instruction having a firstinstruction set format; a processor including a first instructiondecoder logic to indicate to a second instruction decode logic thefunction of each of a plurality of fields within an instruction, whereinthe second instruction decode logic is to use the function of each ofthe plurality of fields indicated by the first instruction decode logicto translate the instruction from the first instruction set format to asecond instruction set format of an extended instruction set, whereinthe second instruction set format is to represent at least one of theplurality of fields with fewer bits than the first instruction setformat of the instruction.
 17. The apparatus of claim 16 furthercomprising an opcode decode logic to decode an opcode within theinstruction into one or more signals to indicate one or more operationsto be performed by executing the instruction.
 18. The apparatus of claim16, wherein the second instruction set format of the instructionincludes an attribute field and an opcode field.
 19. The apparatus ofclaim 18, wherein the attribute field is to indicate an opcode map, aREX value, and a prefix value.
 20. The apparatus of claim 19, whereinthe attribute field of the second instruction set format of theinstruction is to include fewer bits than the sum of the bits of thecorresponding fields of the first instruction set format of theinstruction.
 21. A system comprising: a memory to store an instructionhaving a first instruction format; and a processor including a firstinstruction decoder logic to indicate to a second instruction decodelogic the function of each of a plurality of fields within aninstruction, wherein the second instruction decode logic is to use thefunction of each of the plurality of fields indicated by the firstinstruction decode logic to translate the instruction from a firstformat to a second format, wherein the second format is to represent atleast one of the plurality of fields with fewer bits than the firstformat of the instruction, wherein, the second format of the instructionincludes an attribute field and an opcode field, the attribute field isto indicate an opcode map, a REX value, and a prefix value and in theattribute field, the opcode map is either represented by a first orsecond value, the REX value is indicated by either one bit or four bits,depending on the value of the opcode map, and the prefix value is to berepresented by two bits.
 22. The apparatus of claim 21, wherein thesecond format of the instruction includes a source field to indicate athird source register this different from two other source registersindicated by the instruction.
 23. The apparatus of claim 22, wherein thesecond format of the instruction includes a field to indicate a firstand second vector size.
 24. A method for decoding an instruction in aprocessor, the method comprising: translating a first processorinstruction format having a first plurality of fields into a secondprocessor instruction format of an extended instruction set, the secondprocessor instruction having a second plurality of fields, wherein thesecond processor instruction format retains all information included inthe first processor instruction format using fewer bits, and wherein thefirst processor instruction format includes a prefix field consisting ofeight bits, a REX field consisting of eight bits, and an escape codefield consisting of eight bits.
 25. The method of claim 24, wherein thesecond processor instruction format includes an escape code consistingof eight bits, a rex field consisting of one bit, and a prefix fieldconsisting of two bits.
 26. The method of claim 24, wherein the secondprocessor instruction format includes an escape code consisting of eightbits, a rex field consisting of four bits, a prefix field consisting offive bits, and a prefix field consisting of two bits.
 27. The method ofclaim 24, wherein the second processor instruction format includes athird source register field consisting of four bits and a vector sizefield consisting of one bit.
 28. The method of claim 25, wherein if theinstruction is of a first type, the escape field of the second processorinstruction format includes the value 0xC5.
 29. The method of claim 26,wherein if the instruction is of a second type, the escape field of thesecond processor instruction format includes the value 0xC4.
 30. Themethod of claim 24, wherein the second processor instruction formatincludes an attribute field representing the prefix field, the REXfield, and the escape code field of the first instruction type.